fenv.h

, fenv

Synopsis

Description

Represents the entire floating-point environment. The floating-point environment refers collectively to any floating-point status flags and control modes supported by the implementation.

fexcept_t

Represents the floating-point status flags collectively, including any status the implementation associates with the flags. A floating-point status flag is a system variable whose value is set (but never cleared) when a floating-point exception is raised, which occurs as a side effect of exceptional floating-point arithmetic to provide auxiliary information. A floating-point control mode is a system variable whose value can be set by the user to affect the subsequent behavior of floating-point arithmetic.

The <fenv.h> header defines the following constants if and only if the
implementation supports the floating-point exception by means of the floating-point functions feclearexcept(),
fegetexceptflag(), feraiseexcept(), fesetexceptflag(), and fetestexcept(). Each expands to an integer constant
expression with values such that bitwise-inclusive ORs of all combinations of the
constants result in distinct values.

FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW

The <fenv.h> header defines the following constant, which is simply the bitwise-inclusive
OR of all floating-point exception constants defined above:

FE_ALL_EXCEPT

The <fenv.h> header defines the following constants. Each expands to an integer
constant expression whose values are distinct non-negative values.

FE_DOWNWARD
FE_TONEAREST
FE_TOWARDZERO
FE_UPWARD

The <fenv.h> header defines the following constant, which represents the default floating-point
environment (that is, the one installed at program startup) and has type
pointer to const-qualified fenv_t. It can be used as an argument to
the functions within the <fenv.h> header that manage the floating-point environment.

FE_DFL_ENV

The FENV_ACCESS pragma provides a means to inform the implementation when an
application might access the floating-point environment to test floating-point status flags or
run under non-default floating-point control modes. The pragma occurs either outside external
declarations or preceding all explicit declarations and statements inside a compound statement. When
outside external declarations, the pragma takes effect from its occurrence until another
FENV_ACCESS pragma is encountered, or until the end of the translation unit.
When inside a compound statement, the pragma takes effect from its occurrence until
another FENV_ACCESS pragma is encountered (including within a nested compound statement), or
until the end of the compound statement; at the end of a
compound statement the state for the pragma is restored to its condition
just before the compound statement. If this pragma is used in any other
context, the behavior is undefined.

If part of an application tests floating-point status flags, sets floating-point control
modes, or runs under non-default mode settings, but was translated with the
state for the FENV_ ACCESS pragma off, the behavior is undefined. The default
state (on or off) for the pragma is implementation-defined. (When execution passes from
a part of the application translated with FENV_ACCESS off to a part
translated with FENV_ACCESS on, the state of the floating-point status flags is
unspecified and the floating-point control modes have their default settings.)

Usage

This header is designed to support the floating-point exception status flags and
directed-rounding control modes required by the IEC 60559: 1989 standard, and other
similar floating-point state information. Also, it is designed to facilitate code portability
among all systems. Certain application programming conventions support the intended model of
use for the floating-point environment:

A function call does not alter its caller's floating-point control modes, clear its caller's floating-point status flags, or depend on the state of its caller's floating-point status flags unless the function is so documented.

A function call is assumed to require default floating-point control modes, unless its documentation promises otherwise.

A function call is assumed to have the potential for raising floating-point exceptions, unless its documentation promises otherwise.

With these conventions, an application can safely assume default floating-point control modes
(or be unaware of them). The responsibilities associated with accessing the floating-point
environment fall on the application that does so explicitly.

Even though the rounding direction macros might expand to constants corresponding to
the values of FLT_ROUNDS, they are not required to do so. For
example:

If the function g() might depend on status flags set as a
side effect of the first x+1, or if the second x+1 might
depend on control modes set as a side effect of the call
to function g(), then the application must contain an appropriately placed invocation as
follows: